Urban transmission cables are the lifeline of urban power supply. It is important for grid corporations to ensure the safe and reliable operation of the cables in the daily work. It still remains to be a challenge and manual inspection is difficult to quickly and effectively find and eliminate hidden dangerous points. Therefore, this study proposes a method for intelligently detecting hidden dangerous points of cables based on infrared thermal images. First, using UAV to obtain the infrared map of the outdoor terminal of the cable. Next, the infrared thermal map is binarized by an improved Bernsen algorithm. Then, the projection method is used to extract the subject cables from the binary image in order to eliminate the influence of the background. Finally, according to the intensity chromatogram, the abnormal areas with bright colors in the subject cable image are determined as hidden dangerous points. By applying this method, grid corporations can achieve rapid defect identification, elimination, and fault judgment. It can comprehensively improve the ability to manage and control the status of urban transmission cables.